How is a between-subjects factor different from a within-subjects factor?
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When it comes to a between subjects factor or variable, it means that a different group of subjects is used for each level of the variable. A within-subjects variable, on another hand, is an independent variable that is manipulated by testing each participant or subject at each level of the variable.
Example / Application
Example: Bob Marley Baby
Application: In the video, they show a baby acting frustrated while he’s being buckled up in his baby seat. Parents try to calm him down by turning on Bob Marley song. So, they use a song as a soothing method; this soothing method can be manipulated at different levels. For example, another song can be played next time a baby is fussing. This is not shown in the video, but if parents used a different song to calm the same baby down next time he was acting up, then this would make a soothing method a within-subjects factor. In other words, the same participant would be used for two different levels of independent variable (soothing method). However, if there were another baby whom the parents would try calming down by playing a different song, then we would have a between-subjects design (since two different levels of independent variable were manipulated using two different groups, i.e. babies)
Field, A. (2006). Discovering Statistics Using SPSS: Second Edition. London. Thousand Oaks. New Delhi. Sage Publication